1. Field of the Invention
This invention relates generally to an apparatus and method for anchoring devices in rock material. More specifically, it relates to mine roof bolts and methods of using them to support the rock layer exposed in mine roofs by drilling holes in the roofs and mechanically and adhesively anchoring the bolts to higher layers of rock.
2. Description of the Related Art
Mine shafts sometimes experience cave-ins, collapses, or falling rock due to the layered and stratified makeup of the earth. A mine shaft itself may cause fractures and weaknesses in a strata in its ceiling, or it may just expose an inherently weak and unstable layer. To assist in preserving the integrity of the ceiling, it is common to support the ceiling with bolts anchored up into rock layers above the ceiling. Plates between the bolt heads on the exposed ends of the bolts and the ceilings are used to transfer force from the anchored bolts to the exposed layer of the ceiling. In some applications, the exposed end of the anchored bolt is threaded. Onto these bolts, a nut is threaded, and the nut used to place a preload on the bolt to set an initial lifting force to the plates.
Holes, which are slightly oversized to the bolts, are drilled up into the ceiling. Sometimes the holes must be several feet deep to be sure of anchoring the bolts in a stable layer of rock. Once the holes are drilled, the bolts are inserted into the holes and anchored. There are three methods for anchoring the bolts in the holes, mechanical, adhesive, and mechanically assisted adhesive. This patent relates mostly to the mechanical method or the mechanical aspect of the mechanically assisted adhesive method of anchoring bolts, so the adhesive method will be discussed only briefly before discussing the relevant mechanical art.
Once the hole is drilled, a multi-component adhesive is placed in the blind end of the hole. The components of the adhesive are kept in separate frangible packages to keep them from mixing, for, once they do, a reaction occurs, and the adhesive begins to set up. The components of the adhesive are usually a hardener and a catalyst. When the frangible packages have been placed in the hole, a bolt is inserted and turned rapidly to rupture the packages and thoroughly mix the adhesive components. The adhesive is typically of a fast setting variety and may begin to set after three to five seconds of mixing. For many mechanical anchoring methods, the mechanical anchoring elements on the bolt assist in mixing the adhesive, and the increased resistance to mixing of the setting adhesive activates the mechanical anchoring system.
A very common mechanical anchoring system is shown in U.S. Pat. No. 4,419,805 by Calandra, Jr. This system comprises, basically, a bolt with a threaded end, a camming nut having through its axis a threaded hole to match the bolt, a wooden dowel, and an expansion shell. The camming nut has several sides, the sides being at an angle to the axis of the camming nut to create a wedge effect so that one end of the camming nut is larger than the other. Also, the camming nut has a hole through it transverse to the axis of the camming nut. The diameter of this transverse hole and that of the wooden dowel pin match each other with the length of the dowel pin matching the length of the transverse hole. The expansion shell has at one end a solid ring. The inner diameter of this ring is slightly larger than the bolt diameter. From this ring, several wedge fingers extend in a direction parallel to the axis of the hole. These fingers are equal in number to the sides of the camming nut and, having a wedge shape, taper as they extend away from the ring.
In operation, the expansion shell is placed over the bolt with the tapered wedge fingers pointing up. The wooden dowel is put through the hole in the camming nut, and the camming nut screwed onto the bolt until the dowel pin stops the bolt from passing any further into the camming nut. The expansion shell is pushed up onto the camming nut with the wedge fingers of the expansion shell aligning with the tapered sides of the camming nut. When an anchor hole has been drilled and filled with the adhesive pouches, the bolt is inserted into the hole and turned rapidly to rupture the pouches and mix the adhesive components. The anchoring components on the bolt serve to mix the adhesive. The rapidly setting adhesive provides resistance to the turning of the anchoring elements until the resistance is great enough to cause the bolt shaft to shear the wooden dowel in the camming nut. Once that occurs, the threads begin to pull the camming nut further onto the bolt and into the wedge fingers of the expansion shell. As the camming nut advances into the expansion shell the wedge fingers are expanded out to wedge in the wall of the anchor hole. The wedging of the expansion shell should stop the turning of the bolt before the adhesive sets. Otherwise, as the adhesive sets, a still turning bolt will cause the adhesive to set as small discrete particles as opposed to a single homogeneous anchor. Once the mechanical anchor is set, the bolt can have a preload placed on it. If a mechanical anchor is not used, an operator must wait until the adhesive sets to preload the bolt. So, while the adhesive provides the strongest anchor, the mechanical anchor makes the bolt system more time efficient and therefore more economical.
An additional feature in Calandra, Jr. is the use of a washer to contain the adhesive after the frangible pouches are ruptured and the adhesive is mixed. The washer has an inner diameter closely matching the bolt diameter and an outer diameter approximating that of the hole. It is located below the anchor elements at a position that keeps the adhesive contained in a small enough volume that the adhesive essentially fills the volume. The washer may be fixed in position by a press fit on the bolt or it may be welded in place.
Another common type of mechanical anchor used in mine bolts is the bail type anchor. It has a tapered camming nut and tapered wedge fingers like in the previous type, but the tapered wedge fingers are connected to each other at their thinner upper end by a bail. The bail passes up along the outside of the camming nut and across the top of the camming nut at its wider end. A groove in the camming nut allows the bail to stay within the profile of the camming nut, and in most of these bail type anchors, the wedge fingers are not connected by a ring at their thicker end. In this type of anchor, the resistance of the adhesive causes the camming nut, wedge fingers, and bail to turn more slowly than the bolt, so the bolt begins to advance up through the camming nut until it contacts the bail across the top of the camming nut. At that point, the bolt begins to lift the bail off of the top of the camming nut, and the bail then begins to pull the tapered wedge fingers up toward the camming nut. As the tapered wedge fingers and camming nut become more engaged, the wedging effect between the camming nut, tapered wedge fingers, and the hole sides increases. The bail may break once the camming nut and tapered wedge fingers are sufficiently wedged, if the bolt continues to advance through the camming nut. Once the mechanical anchor is set, a preload is placed on the bolt. Subsequently, the resin fully sets.
Many inventions in this field are directed to additional means for mixing the adhesive as well as the anchoring mechanism. U.S. Pat. No. 4,516,886 by Wright features a bail type anchor that has a two part bail to improve the mixing of the adhesive components. In addition to the bail that passes directly over the camming nut, a second bail extends above the camming nut, effectively providing an elongated hoop to puncture the component pouches and mix the adhesive. The bail that runs directly across the top of the camming nut has a hole through it slightly smaller than the bolt hole in the camming nut. The resistance of the adhesive causes the bolt to force its way through the first bail and advance through the camming nut until the bolt reaches the extended bail which begins to pull the tapered wedge fingers into wedging action with the camming nut and hole sides. Other patents add different mixing means. U.S. Pat. No. 5,042,961, by Scott, fixes a helix shaped length of wire to the bolt below the wedging mechanism, while U.S. Pat. No. 5,073,065, by Kleineke, places an adhesive mixing and retention washer on a tapered shoulder below the anchoring mechanism.
The use of the setting adhesive to drive the wedging action of the mechanical portion of the various anchoring systems has severe drawbacks. Obtaining complete mechanical engagement before the adhesive sets is very time dependent. Variations in the mechanical components, in particular, may prove problematic. The strength of wood shear pins may vary widely. If a sheer pin does not break and the expansion shell is still churning as the adhesive sets, the adhesive may set as small disassociated particles as previously discussed. Once that occurs, the resulting adhesive gravel may provide enough resistance to activate the mechanical anchor, and the bolt may anchor mechanically. However, the adhesive anchor is lost and it is the adhesive anchor that provides the majority of the long term strength of the anchoring system. This is particularly dangerous since the bolt appears to be anchored, but the superior long term anchor of the adhesive component has been lost. Because of the appearance of a good anchor, remedial measures such as placing another bolt immediately nearby are not undertaken. The resulting weakly anchored roof bolt is often called a “spinner” in the mining industry.
Occasionally, if it is obvious to an operator that a mechanical anchor is not actuating, the operator may pause long enough for the resin to nearly set, and then resume turning the bolt. This brings about the destruction of the adhesive, but will pull the bolt up tight for a preload and will give the appearance of a successful anchoring. However, the actual result is a “spinner”.
Some types of rock are particularly soft. This, too, is a problem. The mechanical anchor may widen the hole as it turns and fail to pull tight within the hole. If it continues to turn in a loosened hole, again, the adhesive is at risk.
Another problem is more specific to the mechanical elements of the anchoring systems that use expansion shells having the tapered wedge fingers joined by a common ring at the base with a camming nut being drawn into the expansion shell. These systems typically have four sides on the camming nut and four tapered wedge fingers on the expansion shell with each tapered wedge finger being driven out to the hole wall by a corresponding camming nut side. Sometimes the camming nut will turn within the expansion shell, twisting the wedge fingers to an angle about the axis of the bolt and preventing an effective anchoring in the hole. Again, this substantially decreases the overall holding power of the bolt, allowing ceiling collapses where the load exceeds the strength of the anchor.